1. Field of the Invention
The present invention relates to magnetic random access memory (MRAM) devices. More specifically, the present invention relates to MRAM devices in which a plurality of cells share a common magnetic layer.
2. Description of the Related Art
Magnetic random access memory (MRAM) is a well-known form of memory. In an MRAM device, digital bits of information can be stored as alternative directions of magnetization in a magnetic storage element or cell. The storage elements may be simple structures, such as thin ferromagnetic films, or more complex layered magnetic thin-film structures, such as tunneling magnetoresistance (TMR) or giant magnetoresistance (GMR) elements.
An exemplary TMR memory cell comprises two magnetic layers separated by a barrier layer. One magnetic layer, referred to as the “pinned” layer, has a fixed magnetization direction, whereas the magnetization direction of the other magnetic layer, referred to as the “sense” layer, can be reversed by applying a magnetic field that is not strong enough to affect the magnetization direction of the pinned layer.
A TMR memory cell can operate by allowing a quantum mechanical tunneling of electrons from one magnetic layer to the other through the barrier layer of the cell. The passage of electrons through the barrier layer depends upon the magnetization direction of the sense layer relative to that of the pinned layer. Electrons pass more freely when the magnetic directions of the layers are aligned and less freely when the magnetic directions of the layers are not aligned. Therefore, the state of a TMR memory cell can be determined by observing the degree of electron tunneling through the barrier layer. GMR memory cells operate similarly by sensing current flow or resistance through aligned or anti-aligned magnetic layers, rather than by employing a tunneling dielectric.
A plurality of magnetic memory cells can be organized into an array having any of a wide variety of configurations. One exemplary configuration is a “cross-point” memory array, which comprises a first set of parallel conductive lines covered by an insulating layer, over which lies a second set of parallel conductive lines, perpendicular to the first lines. One set of conductive lines is referred to as the “bit” lines, and the other set of conductive lines is referred to as the “word” lines. The magnetic memory cells can be sandwiched between the bit lines and the word lines at their intersections.